The objectives of this project are to determine the relative importance of genetic vs. environmental factors in various aspects of autonomic functioning and to determine the relationship of autonomic nervous system activity to personality, task performance and electrocortical functioning. Peripheral measures of autonomic functioning, namely skin resistance, heart rate, finger pulse volume, respiration, and skin temperature are recorded during two sessions which include rest periods, a series of mild tones, a reaction time task and mental arithmetic. The subjects are 30 MZ and DZ twin pairs equally male and female. Some opposite sex DZ pairs have also been tested.